Calibri
by Comycat1987
Summary: In his darkest time, Sans found a sliver of light in the depths of a dark lab. However, little did he know reality doesn't allow people to return from the dead, and not everyone can be saved. (Side story to Skeledad, dark, disturbing. Read at your own risk)


**This story is tied to my other fanfic, Skeledad. It's got some spoilers for it, so if you are interested, go check that out first. This can certainly be read on its own, though.**

 **Disclaimer: I do not own Undertale.**

* * *

Who would have known the dark and dreary world that existed under such a bright lab.

It was like shifting between dimensions, a portal to another world. All light and clean air fades as the elevator drawls down with the ugly sound of grating metal.

And who would've guessed Sans would return many times to come.

The short skeleton was so numb, so oblivious to see the wrong in what he was doing. The loss of his father left him with only one feeling- to continue his work and to bring him back.

The first time he entered that lab he was shocked. Holding the cold body of his cat in his hands while entering the TRUE LAB was frightening. All warmth ebbed away from the air like it did as Calibri lived his final moments, and it was as dead as he was.

The TRUE LAB was filled with fake plants, and plain gray tile floors and walls. The walls were lined with black machines that flickered on as they walked past.

"This is where w-we researched. It i-is t-top secret…" said Alphys, the yellow lizard monster in front of him with a white lab coat and round glasses.

Sans merely nodded. He didn't feel like words right now.

There was a room with many beds lined in rows- all the same with pink sheets and white pillows.

"W-we are going to put the f-f-fallen monsters here… and monitor their conditions from here as well." Alphys explained.

They walked to the northern doorway in silence, and made their way down a long, dark hallway.

At the end of the hallway was a large room. To the upper right corner was a huge machine- a red deer-like skull with tubes and wires protruding from it- hanging from the ceiling. Sitting on the ground next to it were numerous jars containing little hearts glowing in various bright colors.

Sans froze.

"Are those…"

"Yes, those are the human souls. That's what we were going to use to obtain the DETERMINATION from the souls." Alphys pointed at the machine.

Sans turned to look at it. It was menacing, shadows creeping across its surface.

Sans felt the grip he had on Calibri grow ever slighter.

Alphys walked over to a shelf near the machine. On it was a rack filled with many syringes, and a single bottle of a light red liquid.

Alphys looked at him, the light glaring off her glasses making it impossible to see her eyes.

"This is DETERMINATION."

…

Sans watched as Alphys carefully stuck the syringe into the bottle, filling it up to a certain mark with DETERMINATION.

He had placed Calibri on the table. Alphys said that the first subject would be his dead cat- if he was okay with that. The idea of getting a part of his broken world back made him say yes.

She walked over to the black cat and slowly put the needle to the cat's neck, injecting the red liquid.

"What is this supposed to do?" Sans asked.

"I… I don't know, really. It could fuel the soul to stay in the body and power it to live on… or not. An animals soul lingers like a humans, but its somewhat lacking in DETERMINATION like ours are. With this power it should become like a human soul."

Sans turned his gaze back to the limp body.

 _I hope so._

…

It was hard to imagine results.

The cat had begun to show signs of life. Basic bodily functions. Breathing. No waking or body movement as far as the data had shown, but it was _living_ again.

The other fallen monsters they had brought in showed similar sighs. Living, but not waking. Sans had come down to the TRUE LAB every day for hours on end. He was beginning to worry Papyrus. But Sans knew that Papyrus would be happy knowing the good cause he was working for… just not yet.

Sans was beginning to regain hope.

…

They woke.

It was nearly 3 am when Alphys had called him, excitedly telling him that Calibri woke up.

Sans ran to the lab as quickly as he could, his soul fluttering.

Indeed, the cat was awake. And quite happy to see him, by the looks of it. Calibri rubbed up against him and purred and kneaded him nonstop, and eagerly accepted the food and water he offered him. Despite literally coming back from the dead, Calibri was in bad shape, and it would take a little bit to recover.

Sans felt tears in his eye sockets as he sat beside the black cat and ran his fingers through his fur- fur he never expected to feel again.

 _You're next, dad._

…

The other experiment monsters were having the same effects. Waking up from near death, recovering… it was a miracle.

Sans's smile was more genuine than it had been in a long time. They had saved Calibri, they had saved the lives of many monsters. Alphys was making preparations to send the monsters home, and Sans was preparing to take Calibri back with him. The cat had recovered phenomenally.

Now it was time to find out if they could save his dad.

…

The next day when he came to take Calibri, Alphys was under the table with her hands on her head as if it would fall off.

"No no no no no no… oh no no no no…" she was sobbing, most of her words inaudible.

Sans felt his soul drop.

 _Something's wrong…_

"Hey, Al, what's wrong? What happened?"

"It all went wrong… it's all wrong… no no no…"

Sans looked around frantically and, without another word, went downstairs.

It was eerily quiet.

 _What could've gone wrong?_

Everyone had been fine. Maybe Alphys made a mistake?

A small meow caught his attention.

Sans hurriedly followed it, into Calibri's room.

Calibri was laying on his sheet, with his head on his paws. His eyes were wide, pupils dilated.

"Hey, what's wrong, Cali?"

Sans approached the cat slowly, blinking even slower to show he meant no harm. The cat merely shifted its gaze up to him and nothing else. It's hindquarters were in an odd position.

As Sans knelt down, he found that Calibri's hindquarters weren't in an odd position..

They were _melting._

The skin had begun to fall away from the bone, leaving a horribly deformed mess of spine and fur. The cat's body was trembling, perhaps from pain or fear.

Sans wanted to throw up. _What happened?!_

As his cat looked up at him with pained eyes, something came to mind… something that had been suggested through research but never proven.

That a vessel injected with DETERMINATION would eventually reject itself.

Calibri meowed again and rubbed his head against Sans's leg.

Sans stared down in complete _horror._

 _What had he done?_

More of Calibri's body was deforming, his ears beginning to drop. He tried to raise his tail, but it had melted and fused with the rest of its deformed hindquarters.

Sans was beginning to shake.

 _Oh no… Alphys was right, it's all wrong… the others must be going through this too…_

Sans began petting the cat's head slowly, a pain ripping through his chest.

"I'm sorry… I'm sorry, Cali… I just wanted to bring you back… I just wanted to see him again…"

Calibri meowed in response, still looking up at him. Despite literally melting, the cat seemed okay now that he was here.

Sans squeezed his eyes shut.

 _I can't let you live like this because I couldn't let go…_

Sans didn't move for a few moments, his body still shaking. His throat hurt.

He scratched behind Calibri's ear.

"Say hi to him for me… and I'm sorry…"

Sans's eye glowed.

In a split second, a bone had shoved its way thought the cat's little chest.

Sans wrapped the cat up in the sheet and left. He left for good and was never going back.

Too bad that was his only bed sheet.

* * *

 **Ugh, so sad :c but that's how it is. The end is a little nod to Sans's lack of a bed sheet in the game. I didn't cry while writing this like I did the end of Skeledad, but I was close.**

 **The writing is better here, but the story sucks :/ it's always one or another.**

 **Thank you for reading, and I hope to see you all on my other stories if you're interested!**

 **I hope you all have a lovely day,**

 **~comycat** **️**


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